1. Field of the Invention
This invention generally relates to an optical waveguide device. More particularly, it relates to a structure of an optical waveguide device in an optical communication system for which particularly high reliability is required, and having a structure that will be used for a high speed optical switch for exchanging multi-channel large capacity data in optical form or will be used for an optical external modulator for ultra-high speed ultra-distance communication, and which improves stability of the modulator and switch for an extended period of time.
2. Description of the Related Art
In ordinary optical waveguide devices used for optical switches and optical modulators, an electric field is applied to an optical waveguide path formed inside a surface of an electro-optical crystal substrate such as lithium niobate (LiNbO.sub.3) so as to change the refractive index of this optical waveguide path. In this way, the switching of optical signals travelling inside the waveguide path, and their phase modulation and intensity modulation are carried out.
In such waveguide devices, a buffer layer of a transparent dielectric film having a smaller refractive index than that of the waveguide path is sandwiched between the waveguide path and the electrode so as to prevent light propagating through the waveguide path from being absorbed by the electrode metal. When an electrode is formed on this buffer layer and a voltage is applied to the electrode, an electric field is applied to the waveguide path formed in the substrate crystal and the refractive index of the waveguide path changes in proportion to the intensity of the electric field. Thus, functions such as switching and modulation are provided.
In this case, the intensity of the electric field applied to the waveguide path and its change with time are greatly affected by characteristics of the buffer layer. Since the optical output changes in proportion to the refractive index of the waveguide path, that is, in proportion to the intensity of the electric field applied to the waveguide path, a technique for accurately controlling the electric field applied to the waveguide path is very important in devices of this kind.
The waveguide devices using such an electro-optical crystal substrate include optical switches, modulators, branching filters, polarized wave controllers, and so forth, but for the safe of convenience the following explanation will be for a Mach-Zehnder type optical modulator using a LiNbO.sub.3 waveguide path for use in a ultra-high speed optical communication modulator.
FIG. 23 shows the appearance of a conventional Mach-Zehnder type modulator. In the drawing, reference numeral 1 denotes a lithium niobate (LiNbO.sub.3) crystal substrate that is cut in such a manner that an X axis of the crystal axis extends in a longitudinal direction of a chip, a Z axis extends in the direction of thickness so as to use an electro-optical coefficient r.sub.33 and a Y axis extends in the direction perpendicular to the X and Z axes. A semi-circular optical waveguide path 2 having a greater refractive index than that of the substrate 1 and having a diameter of about 7 .mu.m is formed on a surface of the substrate 1 by first forming a metal titanium (Ti) film by a known film formation means, such as electron beam deposition, then patterning this titanium (Ti) deposition film into a belt-like form in an X direction shown in the drawing, and thermally diffusing titanium into the substrate 1.
Next, in order to prevent absorption of light propagating through the optical waveguide path 2 by the electrode, silicon dioxide (SiO.sub.2) having a specific dielectric constant of 4.0 and a refractive index of about 1.45 is deposited to a thickness of 0.5 .mu.m over the entire surface of the waveguide substrate 1 by a film formation technique, such as electron beam deposition, thereby forming a buffer layer. (To facilitate an understanding, the optical waveguide path 2 is shown as if it existed on the surface of the buffer layer 3 in FIG. 23.) Furthermore, a signal electrode 4 and a ground electrode 5 consisting of a thin gold (Au) film having a width of 7 .mu.m and a thickness of 10 .mu.m, for example, are formed by vacuum deposition and plating at positions on the surface of the buffer layer 3 corresponding to the optical waveguide path 2. A travelling wave electrode and a signal source 6 are connected by a coaxial cable 7. Similarly, a terminal resistor 8 is connected by the coaxial cable 7. A lithium niobate crystal block 9 is bonded to the end surface of the optical waveguide path 2, and the waveguide path is connected to a fiber 11 by a fiber fixing jig 10.
FIG. 24A shows a sectional structure on a cut line in the modulator shown in FIG. 23. In terms of an electrical equivalent circuit, this section can be expressed, as shown in FIG. 24B, by a buffer layer resistance R.sub.b, the resistance R.sub.LN of the lithium niobate crystal and their capacitances C.sub.B and C.sub.LN. In this equivalent circuit, the voltage V.sub.LN applied to lithium niobate is substantially determined by the capacitance C alone in the equivalent circuit at the instant of the application of voltage to the electrodes 4 and 5, and has a voltage value given by the following equation (1): ##EQU1##
After the passage of a sufficient period of time, V.sub.LN is substantially determined by the resistance R in the equivalent circuit, and is given by the following equation (2): ##EQU2##
Accordingly, the voltage applied to the waveguide path changes between the instant of the application of voltage to the electrodes of the modulator and after the passage of sufficient time. In consequence, the outgoing light from the modulator also changes, which change is referred to as a "DC drift" in lithium niobate waveguide devices.
FIG. 25 is a diagram showing the relation between the impressed voltage and the intensity of outgoing light. In the diagram, when a voltage V.sub.1 is applied, there is an optical output P.sub.1 at the instant of the application, but this optical output decreases with time. Assuming that the optical output reaches P.sub.2, this state is equivalent to the state where only a voltage V.sub.2 is effectively applied to the electrodes, and this drift quantity S can be evaluated by the following equation (3): EQU S=(V.sub.1 -V.sub.2)/V.sub.1 ( 3)
This DC drift is the phenomenon that is generated by the DC component of the voltage applied to the electrodes, and is proportional to the degree of the impressed voltage. In other words, assuming that a 0.3V DC drift occurs when a 1 V voltage is applied, the DC drift of 3 V occurs when a 10 V voltage is applied. It is therefore convenient to express the DC drift quantity by percentage to the impressed voltage when the DC drift is discussed. Accordingly, the DC drift quantity will be expressed by percentage in the following description.
FIG. 26 shows the relation between the resistance R and the capacitance C on the basis of the equations (1) and (2) and the occurrence of the DC drift. When the voltage determined by the resistance of the equation (2) is smaller than the voltage determined by the capacitance of the equation (1), a positive drift occurs and is represented by (a). At this time, the voltage (or the electric field) applied to the waveguide path gradually decreases, and when the voltage determined by the resistance is greater than the voltage determined by the capacitance, a negative drift occurs and is represented by (c). At this time, the voltage (or the electric field) applied to the waveguide path gradually increases. Needless to say, the practical DC drift is not as simple as described above.
Here, the resistance, capacitance, etc., such as the resistance of the interface layer between the LiNbO.sub.3 substrate 1 and the buffer layer 3, the resistance of the buffer layer 3 in the horizontal direction, the resistance arising from the difference of the peripheral portion of the optical waveguide path 2 from the substrate 1, etc., are equivalently expressed by C.sub.B, C.sub.LN, R.sub.B and R.sub.LN. As is known well, the electric resistance of a dielectric (an insulator) changes with the voltage impression time. When mobile ions exist in the buffer layer 3 and in the crystal, a spatial charge distribution owing to their migration must also be taken into consideration. When the DC drift is examined, therefore, these factors must be collectively taken into consideration. However, it is extremely complicated to clarify in detail the mechanisms for all these phenomena and to classify and describe same. Therefore, an explanation will be directed primarily to a method of improving the DC drift characteristics and the characterizing results obtained by such a method.
FIG. 27 shows the evaluation result of the DC drift of the modulator having the prior art structure shown in FIG. 23. FIG. 27 shows the DC drift characteristics changing with time, which are evaluated at atmospheric temperature of 20.degree. C., 60.degree. C., 100.degree. C. and 140.degree. C. It can be appreciated from the diagram that the DC drift is an extremely slow phenomenon occurring at rates of 5% a day at room temperature (20.degree. C.), 30% per 10 days and 100% per 200 days. (In the conventional modulators, too, there is, of course, the case where this phenomenon reaches 100% within several minutes if a process condition is incomplete, such as when there is damage to the crystal.)
Even if the DC drift is such an extremely slow phenomenon, the characteristics of the components for optical communication must be compensated for, for at least a period of 15 years, and the characteristics described above are not sufficient. Furthermore, it is practically difficult to evaluate such a phenomenon in the course of 15 years and then to produce a product, but fortunately, it is known that this phenomenon is accelerated by temperature, as shown in the drawing. In other words, it is known that when evaluation is made at 100.degree. C., the phenomenon can be evaluated in acceleration of 1,000 times at room temperature. It is therefore possible to evaluate and estimate long term characteristics for periods of more than 15 years by carrying out the evaluation at 100.degree. C., and for this reason, the following description will be based on the evaluation result at 100.degree. C. or 140.degree. C. as the reference.
Several methods for improving this DC drift have been proposed in the past. Since the existence of the buffer layer 3 is the main cause for the DC drift as described above, a structure using a transparent electrode for preventing the absorption of propagating light by the electrodes without forming the buffer layer 3 has been proposed (KOKAI (Japanese Unexamined Patent Publication) No. 55-69122). However, there is no material that is transparent at a wavelength of 1.3 .mu.m oar 1.55 .mu.m, which is important for optical communication, and that has a sufficiently smaller refractive index than that of the waveguide path. Accordingly, the structure proposed by the reference described above is not disposed immediately above the waveguide path but is disposed in the proximity of the waveguide path, so as to avoid the problem of optical absorption. In practice, there is a device in which the electrodes must be formed immediately on the waveguide path, such as a Z-cut substrate device. As a counter-measure for such a case, (KOKAI (Japanese Unexamined Patent Publication) No. 61-198133), which reduces the optical absorption by mixing an electrically conductive material with a transparent insulator film has been proposed. According to this method, an optical wavelength range, which is effective as a transparent electrode, can assuredly shift to a longer wavelength side compared to the case where the electrically conductive material is used alone.
To retain the function as the electrode, however, the proportion of this electrically conductive material must be increased, but because generally known conductive materials have strong optical absorptivity at a wavelength of 1 .mu.m or above, it is difficult to form a transparent electrode in this region. Particularly when beams of light pass perpendicularly through the film, they are almost fully absorbed in most cases if the film is used as the buffer layer 3 on the optical waveguide path 2, even though the absorption loss is small. Furthermore in this case, the buffer layer 3 must be divided into the shapes of the electrodes 4, 5 because it fundamentally plays a role equivalent to that of the electrodes.
Another proposal (KOKAI (Japanese Unexamined Patent Publication) No. 1-155631: Article A: Electronics Lett. Vol. 26, No. 17, pp. 1409-1410) contemplates trapping mobile ions by assuming that ions in the buffer layer 3 move and cause localization of ions because of the impressed voltage, and thus generate the DC drift. The introduction of the trap to prevent localization of the ions is common in semiconductor technology. According to the results of the Article A executing this method, an improvement of the DC drift has been attained by doping P. Eventually, however, the stable state can be kept for only two hours, and at least 80% of the DC drift occurs in the course of three hours. FIG. 27 shows the DC drift characteristics of the SiO.sub.2 buffer layer 3 to which nothing is added, according to the prior art structure. Compared to the results of the Article A, the characteristics are much better, and the doping effect of P as the trap cannot be observed.
Furthermore, there has been still another proposal wherein an upper layer of the buffer layer is shaped into a structure where a metal or a semiconductor exists in granular form so as to permit easy injection of electrons into the buffer layer and mitigation of the DC drift (KOKAI (Japanese Unexamined Patent Publication) No. 3-127023). This structure is characterized by its two-layered structure wherein a metal or semiconductor element is locally contained in a granular and metallic state without being oxidized, in an inner electrode of the buffer layer and at an interface portion of the buffer layer. However, the DC drift characteristics of the optical waveguide device fabricated by this method have not yet reached the level of stability required by optical communication systems, as described in Article B (Papers of Electronic Data Communication Society, c-1, Vol. 1. J75-C-1, No. 1, pp. 17-26, January, 1992).
In the field of the optical waveguide devices that operate when an electric field is applied from the electrodes formed on the buffer layer formed on the optical waveguide path formed inside the surface of the opto-electric crystal substrate, to the optical waveguide path, the change of outgoing light with time resulting from the impressed D.C. (direct current) voltage component is referred to as the "DC drift". Although a large number of studies have so far been made to solve this DC drift, no report or data thereby solving this problem has been forthcoming.